An analysis of the biomechanics of interference screw fixation and sheathed devices for biceps tenodesis

Background This study aimed to evaluate the differences in biomechanical properties of biceps tenodesis when performed with sheathed versus unsheathed screws and also to investigate the effect of altering the pre-tension. Methods Tenodesis was performed in an in vitro model using biomechanical test blocks and ovine tendons. Blocks were allocated to 1 of 5 groups which varied by method of tenodesis and cyclical loading protocol: Group A, Biosure PK screw (10-100 N), Group B: 7-8 mm Biosure Sync and Biosure PK screw (10-100 N), Group C: Biosure PK screw (10-70 N), Group D: Biosure PK (20-100 N), and Group E: custom sheath and Biosure PK screw (10-100 N). If tenodeses remained intact after 500 cycles maximum load to failure testing was performed. Findings 30% of tenodeses in Group A failed prior to 500 cycles whereas none failed in the sheathed device groups (P = 0.02). Using a sheathed device prevented mal-rotation. However, tenodeses in Group B were more likely to fail immediately distal to the tenodesis at a load below the anticipated maximum load to failure suggesting tendon damage during fixation. Using the custom sheath, which did not have sharp edges, resulted in a statistically significant increased maximum load to failure in Group E (348 N) when compared to Group A (228 N, mean difference 120 N, P = 0.01) and Group B (253 N, mean difference 95 N, P = 0.0007). Interpretation Sheathed devices prevent mal-rotation and increase stiffness and maximum load to failure. This is further improved by reducing tendon damage at the time of tenodesis.